Nitric oxide as a cellular antioxidant: A little goes a long way

Nitric oxide (NO) is an effective chain-breaking antioxidant in free radical-mediated lipid oxidation (LPO). It reacts rapidly with peroxyl radicals as a sacrificial chain-terminating antioxidant. The goal of this work was to determine the minimum threshold concentration of NO required to inhibit Fe2+-induced cellular lipid peroxidation. Using oxygen consumption as a measure of LPO, we simultaneously measured nitric oxide and oxygen concentrations with NO and O2 electrodes. Ferrous iron and dioxygen were used to initiate LPO in docosahexaenoic acid-enriched HL-60 and U937 cells. Bolus addition of NO (1.5 μM) inhibited LPO when the NO concentration was greater than 50 nM. Similarly, using (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate as a NO donor we found that an average steady-state NO concentration of at least 72 ± 9 nM was required to blunt LPO. As long as the concentration of NO was above 13 ± 8 nM the inhibition was sustained. Once the concentration of NO fell below this value, the rate of lipid oxidation accelerated as measured by the rate of oxygen consumption. Our model suggests that a continuous production of NO that would yield a steady-state concentration of only 10–20 nM is capable of inhibiting Fe2+-induced LPO.